Method for producing low-molecular, highly reactive polyisobutylene

ABSTRACT

A process for preparing low molecular weight, highly reactive polyisobutylene having an average molecular weight M n  of from 500 to 5000 Dalton and a terminal double bond content of more than 80 mol % by polymerization in the liquid phase of isobutene or hydrocarbon streams comprising isobutane [sic] with the aid of a boron trifluoride complex catalyst at from −40 to 20° C. and at from 1 to 20 bar comprises 
     a) polymerizing until the residual isobutene content of the reaction mixture is less than 2% by weight, based on isobutene introduced, or removing residual isobutene towards the end of the polymerization until the residual isobutene content is less than 2% by weight, 
     b) enriching the boron trifluoride complex catalyst which is obtained in the form of droplets in the disperse and/or coherent phase, 
     c) recycling the complex-enriched phases to the polymerization and 
     d) compensating for catalyst losses by adding boron trifluoride and, if necessary, complexing agents.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing low molecularweight, highly reactive polyisobutylene in the liquid phase using aboron trifluoride complex catalyst, the polymerization being conductedsuch that, at the end of the polymerization, the residual isobutenecontent is less than 2% by weight, the complex catalyst is removed andrecycled to the polymerization.

2. Discussion of the Background

Low molecular weight and high molecular weight polyisobutenes havingmolecular weights of up to several 100,000 Dalton have long been knownand their preparation is described, for example, in H. Güterbock:Polyisobutylen und Mischpolymerisate, pages 77 to 104, Springer, Berlin1959. The currently available polyisobutenes of this molecular weightrange are mainly prepared with the aid of Lewis acid catalysts, such asaluminum chloride, alkylaluminum chlorides or boron trifluoride, andgenerally have a molecular weight distribution (polydispersicity [sic])of from 2 to 7.

A distinction must be made between these conventional polyisobuteneshaving average molecular weights of from 500 to 5000 Dalton and thehighly reactive polyisobutenes, which typically have a high vinylidenegroup content of preferably substantially more than 60 mol % and apolydispersity {overscore (M)}_(w)/{overscore (M)}_(n) of less than 2.Such highly reactive polyisobutenes are used as intermediates for thepreparation of additives for lubricants and fuels, as described forexample, in DE-A 27 02 604. For the preparation of these additives,alternating copolymers, in particular polyisobutenylsuccinic anhydrides,are first produced by reacting the terminal double bonds of thepolyisobutene with maleic anhydride, and said copolymers are thenreacted with certain (poly)amines and/or alcohols to give the finishedadditive. Since the vinylidene double bonds are preferred reaction sitesin the ene reaction with maleic anhydride, whereas, depending on theirposition in the macromolecule, the double bonds present further in theinterior of the macromolecules lead to substantially lower, if any,conversion without the addition of halogens, the amount of terminaldouble bonds in the molecule is the most important quality criterion forthis type of polyisobutene.

The polyisobutene cation I formed in the course of the polymerizationreaction may be converted into the corresponding polyisobutene byelimination of a proton. The proton may be eliminated from one of theβ-methyl groups or from the internal γ-methylene group. Depending onwhich of these two positions the proton is eliminated from, apolyisobutene having a vinylidene double bond II or having atrisubstituted double bond III present close to the end of the moleculeis formed.

The polyisobutene cation I is relatively unstable and attempts toachieve stability by rearrangement to form more highly substitutedcations, if the acidity of the catalyst system is high enough. Both1,3-methyl group shifts to give the polyisobutene cation IV andsuccessive or concerted 1,2-hydride group and 2,3-methyl group shifts togive the polyisobutene a cation V may take place. Depending on theposition from which the proton is eliminated, in each case threedifferent polyisobutene double bond isomers can form from the cations IVand V. However, it is also possible for the cations IV and V to undergofurther rearrangement, causing the double bond to migrate further intothe interior of the polyisobutene macromolecule.

All these deprotonations and rearrangements are equilibrium reactionsand therefore reversible, but in the end the formation of more stable,more highly substituted cations and hence the formation ofpolyisobutenes having an internal double bond with establishment of thethermodynamic equilibrium is preferred. These deprotonations,protonations and rearrangements are catalyzed by any traces of acidpresent in the reaction mixture, but in particular by the actual Lewisacid catalyst required for catalyzing the polymerization. Because ofthese facts and since only polyisobutenes having vinylidene double bondsaccording to formula II react very well with maleic anhydride withadduct formation, polyisobutenes of the formula III have in comparisonsubstantially lower reactivity and other polyisobutenes having morehighly substituted double bonds enter into the ene reaction with maleicanhydride virtually only under isomerizing conditions, the continuedefforts of many research groups to find improved processes for thepreparation of highly reactive polyisobutenes having higher and highercontents of terminal double bonds is understandable.

The preparation of low molecular weight, highly reactive polyisobutenefrom isobutene or hydrocarbon streams comprising isobutene, inparticular from C₄ cuts, substantially free from 1,3-butadieneoriginally present therein, from steam crackers, FCC crackers (FCC:Fluid Catalyzed Cracking), i.e. C₄ raffinates, is known from a number ofpatents, for example from EP-A 145 235, EP-A 481 297, DE-A 27 02 604,EP-A 628 575, EP-A 322 241 and WO 93/10063. All these processes relateto the polymerization of isobutene in a single polymerization stage.

A further improvement is provided by the two- or multi-stage process ofWO 96/40808, which comprises carrying out the polymerization reaction inat least two polymerization stages, the added isobutene at asubstantially constant isobutene concentration being polymerized to apartial conversion of up to 95% in the first polymerization stage andthe polymerization of the remaining isobutene being continued in one ormore subsequent polymerization stages, without or after prior isolationof the polyisobutene formed in the first polymerization stage.

In addition to the efforts to optimize the process as described in theabovementioned publications, it was also attempted to recover BF₃ foreconomic and ecological reasons. Thus, EP-A 0 742 191 suggests thethermal decomposition of the BF₃ complex in the product stream and theabsorption of the liberated BF₃, for re-use, in an olefin streamcomprising a promoter.

This process has the disadvantages that the product of value issubjected to thermal stress in the presence of the catalyst whichresults in isomerization of the vinylidene double bond to give the morehighly substituted double bond type, and that the wastewater is pollutedby the complexing agent. The suggested method is not practical for thepreparation of reactive polyisobutenes.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process whichmakes it possible to prepare highly reactive polyisobutylene and torecycle the catalyst.

We have found that this object is achieved by a process for preparinglow molecular weight, highly reactive polyisobutylene having an averagemolecular weight M_(n) of from 500 to 5000 Dalton and a terminal doublebond content of more than 80 mol % by polymerization in the liquid phaseof isobutene or hydrocarbon streams comprising isobutene with the aid ofa boron trifluoride complex catalyst at from −40 to +20° C. and at from1 to 20 bar, which comprises

a) polymerizing until the residual isobutene content of the reactionmixture is less than 2% by weight, based on the total amount of streamsintroduced, or removing residual isobutene towards the end of thepolymerization until the residual isobutene content is less than 2% byweight,

b) enriching the boron trifluoride complex catalyst which is obtainedhere in the form of droplets in the disperse and/or coherent phase,

c) recycling the complex-enriched phases to the polymerization and

d) compensating for catalyst losses by adding boron trifluoride and, ifnecessary, complexing agents.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a reactor for the preparation of polyisobutylene.

DETAILED DESCRIPTION OF THE INVENTION

The polymerization can be carried out in the presence or absence ofsolvents. Solvents used are generally hydrocarbons, e.g. butane,pentane, heptane, octane, which can be employed in pure form or in theform of technical grade mixtures.

To achieve as far as possible a quantitative separation of the complex,preference is given to using complexes having a limited solubilityand/or cooling down the reaction mixture to, for example, from 5 to 30°C. below the temperature of the reactor, preferably by from 10 to 20° C.

For the purposes of the present invention, complexes having limitedsolubility are complexes having a solubility in, for example, 1:1hexane/polyisobutene at −15° C. of from 0.1 to 10 g/l, preferably from0.3 to 3 g/l.

Complexes having a limited solubility may be obtained by the choice ofthe complexing agent. For instance, a methyl tert-butyl ether complex isless soluble than the isopropyl tert-butyl ether and a methanol complexis less soluble than an isopropanol complex. It is also possible toachieve an appropriate solubility by mixing the complexing agents.Furthermore, water released by incomplete drying of the isobutene feed,especially the fresh isobutene feed (the recycled isobutene is generallydried) and/or by secondary reactions, such as dehydration of the alcoholcocatalyst or cleavage of any alkyl ethers used as cocatalysts, may forma BF₃/water complex which accumulates owing to the recycling. The degreeof separation is generally determined by these amounts of water. Thesolubility of the BF₃ complex is also reduced by methanol, hydrofluoricacid, formic acid, formaldehyde and acetaldehyde. Preference is given tousing boron trifluoride complexes or complex mixtures with water,methanol, ethanol, n- or isopropanol, 2-butanol or tert-butanol or thetert-butyl ethers thereof, particular preference being given to borontrifluoride complexes with methanol, ethanol, n- or isopropanol,2-butanol or the tert-butyl ethers thereof.

In any case, the polymerization continues during separation of the BF₃complex from the polymerization mixture, and, depending on polymer andisobutene concentration, the polyisobutene formed may be unfavorablyaltered with respect to molecular weight, molecular weight distributionand reactivity. The separation is therefore conducted at low isobuteneconcentrations, i.e. less than 2% by weight, preferably less than 1% byweight, based on the total amount of streams introduced, so that theproperties of the resulting polymer are not significantly altered.

For this reason, stage (a) of the invention is preferably operated inthe multi-stage mode of the polyisobutene process described in WO96/40808, where residual isobutene from the main reactor is consumed inthe post-reactor to a level of about 0.5%. With homogeneous catalysis inthe main reactor, there is also increased complex separation by thecooled post-reaction. The solubility of the complex is reduced by almostone power of ten, especially if the temperature is also reduced.

Since separation of some of the very finely divided complex droplets ofthe resulting emulsion is quite difficult, the emulsion should have alow viscosity. An advantageous technical solution is provided bycoalescing filters or separators including large-volume separators. Inthis way, from 40 to 95% of the catalyst can be separated withoutproblems and returned to ink the reactor.

If the complex has a particularly high separation rate and the reactorfeeds are dried incompletely, a partial discharge of, for example, from10 to 30% of the separated complex may be necessary. If this is to beavoided, it is advisable to dry the reactor feeds over a 3 Å molecularsieve at advantageously from 3 to 5° C.

It is also possible to control the isobutene concentrations according tothe invention by flashing the reactor effluent. This involves flashingthe polymerization mixture at below 0° C., generally at the reactortemperature, into a flash vessel which has a pressure lower than that inthe polymerization reactor which is generally from 1 to 20 bar abs,preferably from 1 to 10 bar, particularly preferably from 1 to 6 bar.The pressure in the flash vessel is generally 1-100 mbar, preferably5-50 mbar, particularly preferably 10-30 mbar abs. The pressure in theflash vessel is controlled so as to achieve the desired residualisobutene concentration. The vapors usually have an acid number (AN) inaccordance with DIN 53402 in mg of KOH/g of substance of up to 1.0 andare generally condensed at low temperatures, preferably at the reactortemperature, and recycled to the reactor. Owing to the relatively lowtemperature and the relatively low isobutene content, a water-containingcomplex precipitates in the bottom product of the flash evaporationwhich is separated in a phase separator and/or coalescer and recycled tothe reactor.

In a particular embodiment, the present process is an emulsionpolymerization. In emulsion polymerization, the complex is no longercompletely soluble in the reaction mixture even at the start of thepolymerization. Undissolved complex droplets generally increase theformation of oligomers (distillate yield) unless the undissolved complexdroplets are very well dispersed. Such a dispersion of the undissolvedcatalyst droplets is achieved, for example, by means of a circulatingpump in a high-output loop reactor as described in U.S. Pat. No.5,286,823. Toothed-wheel mixers or other devices having a high energyinput are also suitable.

After the reaction, the droplets are separated in separators, with orwithout using coalescing aids. In many cases, a complex separation andrecycle of up to 95% is achieved.

However, preference is given to homogeneous polymerization.Specifically, the homogeneous polymerization is conducted in the liquidphase with the aid of a boron trifluoride complex catalyst at from −40to 20° C., preferably at from −30 to 10° C., especially at from −20 to0° C., and generally at from 1 to 20 bar, preferably at from 1 to 10bar, especially at from 1 to 6 bar, in a manner known per se, e.g.according to U.S. 5,408,018 and 5,286,823, which are incorporated hereinby reference.

The preferred operating procedure for stage (a) of the process of theinvention is, as mentioned above, the multi-stage method described in WO96/40808.

In its simplest embodiment, the polymerization according to this methodis operated in two polymerization stages. Various methods can be adoptedin order to obtain high terminal double bond contents of thepolyisobutene and a low fluorine content of the washed polymer solution.

For example, it is possible to establish an isobutene conversion of from5 to 98%, preferably from 50 to 95%, in particular from 50 to 90%, inthe first polymerization stage and to complete the polymerization in thesecond stage.

The second polymerization stage is advantageously operated at the sameor a lower polymerization temperature than the first polymerizationstage, the temperature difference usually being from 0 to 20° C.,preferably from 0 to 10° C.

Since the polymerization of the isobutene is exothermic, thepolymerization temperature in the first polymerization stage iscontrolled, at a predetermined coolant temperature (the coolant isadvantageously liquid ammonia, but other coolants, such as liquid sulfurdioxide or aqueous salt solutions or alcohol/water mixture can also beused), by the reactivity of the catalyst complex used, i.e. by theaddition of complexing agent at a rate such that said polymerizationtemperature remains essentially constant, apart from technicallyunavoidable fluctuations or sudden variations in concentration at thepoints of introduction. The isobutene conversion in the firstpolymerization stage is controlled by adjusting the reactivity of thecatalyst complex via metering of the complexing agent, taking intoaccount the abovementioned parameters, i.e. coolant temperature,polymerization temperature and an average residence time of the reactionmixture in the reactor.

The discharge from the first polymerization stage is preferably passeddirectly into the second polymerization stage. Here, the polymerizationis carried out, without the addition of fresh isobutene, preferably at alower polymerization temperature than that in the first polymerizationstage. This can be effected by means of a lower coolant temperature orwith the use of a coolant at the same temperature as in the firstpolymerization stage, for example with the use of cooling apparatus usedthere, by controlling the cooling in such a way that the quantity ofheat removed from the polymerization mixture is greater than thequantity of heat released there in the polymerization of the remainingisobutene. Under certain circumstances, it may be necessary oradvantageous to replenish or to activate the complex which is moreinactive as a result of lower temperatures by adding boron trifluorideso that the polymerization does not stop prematurely. This addition ofboron trifluoride can be carried out before or after the introduction ofthe polymerization mixture into the second polymerization stage. Theseparation of the complex is improved by subsequent activation withoutdeterioration in product quality.

With adjustment of an isobutene conversion of from 50 to 90%, theresidence time of the polymerization mixture in the first polymerizationstage is usually from 5 to 60 minutes, but may be shorter or longerdepending on whether a very active or a less active catalyst is used. Inthe second polymerization stage, a residence time of from 1 to 180,preferably from 2 to 120, minutes is generally established. In thesecond polymerization stage, the isobutene conversion is generallyadjusted such that the total conversion of the isobutene in the firstand second polymerization stages is in general from 80 to 100%,preferably from 90 to 100%, in particular from 95 to 100%.

If the polymerization mixture in the second stage contains more than 2%by weight of isobutene, based on the isobutene introduced into the firststage, the unconverted isobutene can alternatively be fed, together withthe polymerization discharge from the second polymerization stage,without further working up, to a third polymerization stage and furtherpolymerized there, at a polymerization temperature which is lower thanthat in the second polymerization stage, to give an isobutene content ofless than 2% by weight. In general, the polymerization temperatureestablished in such a third polymerization stage is from 0 to 20° C.,preferably from 0 to 10° C., lower than the polymerization temperaturein the preceding second polymerization stage. The polymerizationtemperature can be established using the measures described above forestablishing the polymerization temperature in the second polymerizationstage. The residence time of the polymerization mixture which isestablished in the third polymerization stage depends on the catalystactivity and on the desired conversion and is generally from 2 to 180,preferably from 10 to 120, minutes. As stated in the explanation forcarrying out the second polymerization stage, it may be necessary oradvantageous to increase the catalyst activity by adding borontrifluoride. The pressure in the second and, if appropriate, thirdpolymerization stage is generally from 1 to 20 bar, preferably from 1 to10 bar, especially from 1 to 6 bar.

Although the use of second and third polymerization stages isadvantageous also when pure isobutene is used in the polymerization, itproves to be particularly advantageous when C₄-hydrocarbon streamscomprising isobutene, such as C₄ raffinates or C₄ cuts from thedehydrogenation of isobutene, are used as starting material in theprocess of the invention, since, as a result of said hydrocarbonstreams, isobutene losses are avoided, there is no increase in the levelof undesirable hydrocarbons due to recycling of unconverted isobutenecomprising other hydrocarbons into the first polymerization stage andconsequently a high-quality, virtually fluorine-free, low-isobuteneraffinate II is obtained in addition to polyisobutene (PIB) having ahigh terminal double bond content. The polymerization discharge from thethird stage can be worked up in the same way as that described forworking up the discharge from the second polymerization stage.

As regards details on the implementation of the multi-stagepolymerization, reference is made to WO 96/40808, which is thereforeincorporated herein by reference. Stage (b) comprises the enrichment ofthe catalyst complex which is precipitated in the form of droplets andwhich first forms as a disperse phase. In general, some of thesedisperse droplets quite quickly also form a coherent phase whichcomprises solvent, isobutene and lower oligomers and sometimes alsodispersed polymer. The complex droplets have a significantly higherdensity than the polymer solution and, when the droplets aresufficiently large, a coherent complex phase is quickly formed as thelower layer in separators or collecting tanks. In terms of shape andequipment, the tanks are preferably adapted to the problem to be solved,i.e. they are provided, in the lower part, with an interfacial layermeasuring device and a teat or taper related to the amount of complexseparated per unit time. Owing to the risk of side reactions, thecoherent complex phase preferably has a volume corresponding to thecomplex consumption in the polymerization of at most 1-3 hours.

However, a greater or lesser fraction of the complex droplets is sosmall that the separation may be improved by longer residence times ofthe upper phase high in polymer and low in complex or other technicaldevices. Different residence times for the upper and lower phases arecontrolled via the position of the interfacial layer, whereas theresidence time itself is controlled via the size of the tank, theamounts of feed added and the position of any liquid level. Typicaldevices for increasing the size of the complex droplets and consequentlyfor improved separation are coalescers, i.e. filtering devices whichconvert the small, hydrophilic complex droplets into larger complexdroplets via pore diameter and/or hydrophilic filter material. Commonlyused hydrophilic filter materials are glass fibers, phenolic resins orphenolic resin coatings, but acrylonitrile fibers or coatings can alsobe used here, although they have another function or effect at thispoint than in the adsorption described below. Said coalescence is oftenfacilitated by a separator, in this case a hydrophobic filtration. Ahydrophobic filter material, if necessary in combination with a narrowpore diameter, prevents the passage of finely dispersed catalystdroplets.

As a result of such a treatment, the upper phase high in polymer isgenerally homogeneous and only contains soluble complex fractions.However, at prolonged residence times following this treatment,post-reaction may again lead to turbidity, i.e. the formation of adisperse complex phase. However, the complex separation is generallyimmediately followed by an extraction to remove soluble complex.However, in a particularly preferred embodiment, the dissolved complexis adsorbed on nitrile-containing materials, for example as described inEP 791 557, or on nitrile-modified silica gel. Preference is given tousing the nitrile-containing polymers in fiber form, for examplepolyacrylonitrile fibers, since a high surface area can be obtained by acorresponding spinning process.

The complex catalyst is liberated from the loaded nitrile-containingmaterials by thermal treatment and recycled to the polymerization.

As an alternative to enriching or separating the complex droplets bymeans of the abovementioned filtering devices, the disperse complexdroplets can be coalesced and thus enriched in the coherent phase andseparated by means of electrostatic coalescers as described, forexample, in Chem. Ing. Techn. 62 (1990) 525. Such a process is describedin Malaysian patent application PI 9704367 for the separation of ioniccobalt salts from hydroformylation mixtures of polyisobutene. It wassurprising that this separation can also be effected with the borontrifluoride complexes used in the process of the present invention,since these complexes are electrically neutral.

However, the formation of a coherent phase comprising the catalystcomplex is not strictly necessary for recycling. Additional recycling tothe reactor of a phase in which the complex is still dispersed is alsopossible, if desired. In this case, the separator is not operated withan interfacial layer, and a partial polymer recycle into the reactor isaccepted.

According to stage (c) the enriched and/or separated catalyst complex isthen recycled, usually without further purification, to thepolymerization, in the case of a multi-stage polymerization generallyinto the first stage. It is generally possible to recycle from 70 to 90%of the catalyst complex.

Even if the catalyst complex is virtually completely recycled, there issome loss of activity, which is compensated for by adding small amountsof BF₃ catalyst, for example from 1 to 30, preferably from 3 to 20, inparticular from 5 to 10, % by weight, based on the proportion of BF₃complex needed in straight passage, and boron trifluoride. Theproportion of complexing agent depends on the complex losses and maypossibly be less than the stoichiometric amount or even fall to zero, ifthe hydrocarbon starting material comprises, for example, complexingagents such as methanol which are not held back by the upstreammolecular sieve.

It was surprising that recycling of all the complex including the lesspreferred complexing agents water or methanol and not only recycling ofthe boron trifluoride set free from the complex is possible withoutadversely affecting the properties of the polyisobutene prepared withit.

EXAMPLE 1

The reactor W1 according to FIG. 1 consists of a Teflon tube which has alength of 7.6 m and an internal diameter of 4 mm and via which 50 l/h ofreactor content are circulated by means of a gear pump P1. The tube andpump have a capacity of 100 ml. The Teflon tube and pump head areimmersed in a cold bath at −19° C. (Kryostat). Isobutene (line 1) andn-hexane (line 2) are used as feed, at a rate of 140 g/h of isobuteneand 160 g/h of hexane. The feed is dried over a 3 Å molecular sieve to awater content of less than 3 ppm (K1) and fed to the loop reactorthrough a capillary which has an internal diameter of 2 mm and isprecooled to −19° C. BF₃ (line 3) and isopropanol as complexing agent(line 4) are directly introduced into the loop reactor. The amounts ofBF₃ and isopropanol are varied until a molecular weight M_(N) of 1000 isobtained at an isobutene conversion of 90%. The amount of BF₃ added inthe steady state is 10 mmol/h and the amount of isopropanol added is13.5 mmol, at a reactor temperature of −13° C.

The isobutene conversion is determined by gas chromatography usingn-hexane as reference. The feed, the reactor volume and the volumecontraction due to polymerization give an average residence time of 13minutes. The polymerization is terminated with 15 ml/h of acetonitrile(line 5) immediately after the pressure regulation means.

The pressure conditions in the reactor are determined by its geometry,the amount circulated, the viscosity of the reaction mixture and thepressure regulation. The pressure regulation means directly at thereactor outlet on the pressure side of the pump is set to 7 bar and,under the prevailing concentration conditions, about 4 bar are measuredon the suction side of the pump P1. The pressure loss of the system isthus 3 bar. After termination of the reaction by means of acetonitrile(stream 5), the reactor discharge is stirred with 600 ml/h of hot water(60° C.) (line 6) in a 1 l stirred flask R1. A sample of the reactorcontent (taken at C, path A set) is titrated in aqueous phase withethanolic KOH using phenolphthalein as an indicator until the pink coloris visible for 10 minutes. An acid number of 2.6 is thus measured.

To establish a steady-state equilibrium in the polymerization reactor,the stirred flask is disharged after 2 hours and a sample is collectedover a period of one hour, the aqueous phase of this sample is separatedand the organic phase is worked up by distillation. The distillation iscontinued at 230° C. until a pressure of 2 mbar_(abs) is reached, theoligomer content of the distillate is determined by gas chromatographyand the bottom product is characterized. The amount of terminal doublebonds (vinylidene content) was determined using ¹³C-NMR by known methodsand was found to be 88%. The viscosity at 100° C., measured in anUbbelohde viscometer, was 203 mm²/s, the molecular weight M_(N) was1005, as determined by GPC, and the molecular weight distribution D was1.5. The bromine number is 16.0, and the polymer yield is 97%.

At this time, a post-reactor as described in WO 96/40808 in the form ofa stainless steel capillary having an internal diameter of 2 mm and alength of 5 m (W2) and a flooded 400 ml stainless steel tank B1 havingan inlet via a tube inserted into the tank bottom and upper and loweroutlets are started up downstream of the pressure regulation means andunder the conditions described above. To this end, the path via line Ais closed and the path is set via line B. Both devices are immersed intothe same cooling bath as the main reactor. A Teflon filter having a porediameter of 0.2 μm is located in the upper exit of the tank B1, whereasthe lower exit leads to a pump which is able to recycle materials intothe main reactor. The pressure regulation at the exit of the mainreactor is reduced by 1 bar, and the termination using acetonitrile isomitted. Now the tank B1 fills up. The reaction proceeds in the tank B1,and a temperature rise of 4° C. is observed, with the isobuteneconversion rising to 99.4%. After about 45 minutes, the tank B1 is full,and a sample is taken in the stirred flask R1 and worked up as describedabove.

This gives a polymer yield of 95%, a bromine number of 16, an M_(N) of998, a molecular weight distribution of 1.5 and a reactivity of 87%,i.e. only minor variations, if any, compared with the originalprocedure. However, the acid number of the feed to the stirred flask(sampling at C) is only 0.2. This corresponds to 92% separation of thecomplex remaining in the tank B1. It contains about 20% of water and isdrained off.

At this point, the complex recycle pump P2 is switched on at a rate of2.5 ml/h, which inevitably leads to a small amount of the organic phasebeing recycled, because the amount is considerably larger than theamount of complex separated. At the same time, the BF₃ feed is reducedto 1 mmol/h and the isopropanol feed is reduced to 0.4 mmol/h. Theincreased amount of BF₃ is necessary to keep the reaction going so thatthe isobutene conversions in the main reactor, the post-reactor and thetank B1 remain constant. The product data remain constant, the polymeryield is reduced to 94%.

EXAMPLE 2

The experiment is conducted as described in Example 1, except that thepost-reactor W2 is replaced by a flash vessel having a volume of 200 mland operated at 20 mbar_(abs). The vapors are condensed by means of adry ice cooler and recycled into the main reactor by a pump, and thefeed of fresh hexane is reduced by 20 ml. The flash vessel bottoms arepumped into the tank B1 where the complex is separated as describedabove. The acid number of the crude product obtained is 0.1, the polymeryield rises again to 96%, and the polymer properties are virtually thesame as in the procedure without post-reactor W2 as described in Example1.

EXAMPLE 3

The procedure described in Example 1 is repeated, except thatmethanol/water is used as complexing agent. The reactor is fed with amixture of 5 mmol each to obtain similar conversions in the main reactorand the post-reactor W2. In the steady state, the isobutene conversionobtained is 90% when path A is set. Downstream of the filter exit B1 viapath B, the acid number is only <0.1, i.e. the separation rate is morethan 95%. Washing and distillation yields a polymer having a vinylidenedouble bond content of 83% and a polymer yield of 94%, an M_(N) of 1010,a molecular weight distribution of 1.6 and a viscosity of 218 mm₂[sic]/s.

Then the complex recycle system as described in Example 1 is started.After one minute, water and methanol feeds are turned off and a feed ofabout 1 mmol of BF₃ is continued to maintain an isobutene conversion of90%. The properties of the product remain constant after a further 16 hof operating time.

We claim:
 1. A process for preparing low molecular weight, highlyreactive polyisobutylene having an average molecular weight M_(n)ranging from 500 to 5000 Dalton and a terminal double bond content ofmore than 80 mol % by polymerization in the liquid phase of isobutene orhydrocarbon streams comprising isobutene with the aid of a borontrifluoride complex catalyst which is a boron trifluoride complex or acomplex mixture with water, methanol, ethanol, n- or isopropanol,2-butanol or tert-butanol or the tert-butyl ethers thereof, at atemperature ranging from −40 to +20° C. and at a pressure ranging from 1to 20 bar, which comprises: a) polymerizing isobutene and optionallyremoving unreacted isobutene to provide a reaction medium having acontent of unreacted isobutene of less than 2% by weight, based on thetotal amount of streams introduced, b) enriching the boron trifluoridecomplex catalyst which is present in the reaction medium in the form ofdroplets in the disperse and/or coherent phase, which forms aspolymerization proceeds in step a); c) separating a boron trifluoridecomplex-enriched phase from the reaction medium; d) recycling the borontrifluoride complex-enriched phase to the polymerization phase of theprocess; and e) compensating for catalyst losses incurred by theseparation and recycling of catalyst material by adding borontrifluoride to the polymerization medium.
 2. The process according toclaim 1, wherein, in the compensating step e), complexing agent is addedto the boron trifluoride catalyst at the polymerization step to complexboron trifluoride.
 3. The process according to claim 1, wherein thepolymerization reaction is conducted mainly in the homogeneous phase. 4.The process according to claim 1, wherein the unreacted isobutenecontent of the reaction medium is less than 1% by weight.
 5. The processaccording to claim 1, wherein a sparingly soluble boron trifluoridecatalyst is added to the enriched boron trifluoride complex catalystphase in order to increase separation of the catalyst.
 6. The processaccording to claim 1, wherein the reaction mixture is cooled to increaseseparation of the boron trifluoride complex.
 7. The process according toclaim 1, wherein from 70 to 90% of the amount of catalyst complexseparated from polymerization medium (a) is recycled to fresh startingisobutene polymerization medium.
 8. The process according to claim 1,wherein the unreacted isobutene content in the polymerization reactionmedium is controlled by flash evaporation under reduced pressure.
 9. Theprocess as claimed in claim 1, wherein the polymerization is conductedin multiple stages until the unreacted isobutene content is less than 1%by weight.
 10. The process as claimed in claim 1, wherein thepolymerization is conducted in two phases in an emulsion polymerizationreaction.
 11. The process as claimed in claim 1, wherein the unseparatedportions of complex catalyst remaining after removal of the separatedcomplex catalyst are deposited on nitrile-containing fibers ornitrile-modified support material, the catalyst is liberated from thefibers or support material by thermal treatment and the catalyst isrecycled to the polymerization reaction medium.
 12. The process asclaimed in claim 1, wherein the boron trifluoride complex catalystdistributed in the polymerization mixture in dispersed form is separatedby means of an electrostatic coalescer.
 13. The process according toclaim 1, wherein the polymerization reaction is conducted in multiplestages until the unreacted isobutene content is less than 1% by weightin the reaction medium.